(1) Field of the Invention
The present invention relates to an aircraft provided with a tail plane including means for combating flutter.
(2) Description of Related Art
It is common practice to provide the tail zone of an aircraft with stabilizer members, referred collectively as a tailplane. For example, it is known to arrange on a tail boom a first tail member that is substantially vertical relative to the aircraft and a second tail member that is substantially horizontal relative to the aircraft, when on the ground. Under such circumstances, the first tail member is conventionally referred to as a “tail fin” or “lateral stabilizer”, with the second tail member conventionally being referred to as a “tail plane” or indeed a “horizontal stabilizer”.
The tail plane may comprise an airfoil surface passing right through the tail boom in a transverse direction. The airfoil surface then has a first stabilizer portion and a second stabilizer portion extending on either side of the tail boom, and optionally a central portion extending inside the tail boom, the central portion connecting the first portion to the second portion. Such a tail plane is sometimes referred to below as a “through” tail plane insofar as the airfoil surface includes an element passing through the tail boom.
Under such circumstances, the tail boom has first and second orifices, each arranged in a respective side wall of the tail boom. In order to position the tail plane, the airfoil surface is inserted through the tail boom via the first orifice, the airfoil surface then passing out through the second orifice until it reaches its final position. It can be understood that first assembly clearance is provided between the first portion of the airfoil surface and the first orifice, and that second assembly clearance is provided between the second portion of the airfoil surface and the second orifice.
Furthermore, fastener systems are used to keep the airfoil surface in position relative to the tail boom.
The fastener system is then provided with at least one projecting rim of the dropped edge type in register with each stabilizer portion of the airfoil surface or with a metal fitting. The projecting rim then co-operates with screw-fastener means for fastening the tail plane.
Among the aerodynamic phenomena encountered by a tail plane in flight, there is the phenomenon of aeroelastic flutter, commonly referred to simply as “flutter”, which phenomenon is caused by the stream of air moving around the tail plane.
In order to understand the phenomenon, it is important firstly to realize that even in the absence of vibration, a tail plane is subjected to variation in twisting moments in each of its sections due to vertical movement of the tail plane, and consequently it is subjected to variation in lift. This observation remains when vibration is also present.
It should be recalled that a tail plane has its own natural modes of vibration both in flapping and in twisting. In particular, these are a natural mode of vibration symmetrical flapping, a natural mode of vibration asymmetrical flapping, a natural mode of vibration symmetrical twisting, and a natural mode of vibration asymmetrical twisting. The adjective “symmetrical” is used of a natural mode of vibration when the first stabilizer portion and the second stabilizer portion of the airfoil surface are vibrating in phase, while the adjective “asymmetrical” is used when the first stabilizer portion and the second stabilizer portion of the airfoil surface are vibrating in phase opposition.
The flutter phenomenon may occur when the natural frequency of a natural mode of vibration in flapping and the natural frequency of a natural mode of vibration in twisting come close together under the effect of aerodynamic stresses. The term “frequency” is used below for convenience to designate the natural frequency of a natural mode of vibration.
This constitutes an alarming situation that may be encountered when the speed of an aircraft increases so as to become relatively high. When said speed reaches a critical speed, also referred to as the flutter speed, the damping of the natural mode of vibration in flapping and/or of the natural mode of vibration in twisting, as provided by the airstream runs the risk of becoming negative. Under such circumstances, a stabilizer portion of the tail plane runs the risk of being excited at a frequency of vibration in flapping or in twisting that is close to said first and second frequencies. The stabilizer portion then deforms in a natural mode of vibration in flapping and a natural mode of vibration in twisting. It is important to understand that this phenomenon is likely to diverge, i.e. to vary rapidly and lead to an explosion of the stabilizer portion concerned.
In other words, the tail plane vibrates in a vertical plane while simultaneously performing vertical movement in twisting. These two movements together, coupled with the airstream that represents a source of energy, can lead to dynamic instability. The twisting excites flapping, and flapping then reinjects energy into twisting. The flapping and twisting modes of vibration become amplified giving rise to said dynamic instability or “flutter”. This phenomenon is very dangerous, with the tail plane often breaking quite suddenly, making it impossible to carry out any avoidance maneuver.
In order at least to limit the risk of breakage due to flutter in a stabilizer portion of a tail plane, it is possible to introduce damping. Damping may be provided by means of a suspended mass arranged at a free end of a stabilizer portion or by means of a damping material, for example, in order to prevent the damping in question going negative in the authorized range of speeds for the aircraft.
Nevertheless, it can be difficult to demonstrate that a damping scheme is effective over time, i.e. that it guarantees some minimum level of damping. In addition, it is clear that any solution that consists in adding mass penalizes the aircraft from a weight point of view.
Under such circumstances, it is also possible to envisage modifying the actual structure of the tail plane. However, that technique can be expensive if it is desired to optimize existing aircraft, insofar as it involves replacing the tail planes in question.